1. Field of the Invention
The present invention relates to self-cleaning surfaces which have an artificial, at least partly hydrophobic surface structure comprising elevations, the elevations being formed by hydrophobic particles fixed to the surface by means of a substrate, said surfaces having improved mechanical strength, a process for the production thereof and the use thereof.
2. Description of the Related Art
Articles having surfaces which are extremely difficult to wet have a number of commercially important features. The most commercially important feature is the self-cleaning effect of surfaces which are difficult to wet, since the cleaning of surfaces is time-consuming and expensive. Self-cleaning surfaces are therefore of very great commercial interest. Adhesion mechanisms are as a rule attributable to interfacial energy parameters between the two surfaces in contact. The systems attempt to reduce their free interfacial energy. If the free interfacial energies between two components are already very low by nature, it may generally be assumed that the adhesion between these two components is weak. What is important is the relative reduction of the free interfacial energy. In the case of pairs having a high and a low interfacial energy, very often the possibilities of interactions are important. Thus, for example on application of water to a hydrophobic surface, it is not possible to bring about a marked reduction of the interfacial energy. This is evident from the fact that the wetting is poor. Applied water forms drops having a very high contact angle. Perfluorinated hydrocarbons, e.g. polytetrafluoroethylene, have very low interfacial energy. In numerous cases, however, wetting of the surface by water is not desired. Thus, wetting of surfaces by water often leads, after run-off, to residues of dissolved or suspended solids as unattractive residues on the surface. This problem occurs in particular in the case of surfaces which are exposed to rainwater and splashes.
Surfaces which are difficult to wet with water are known. It is known that, for achieving a good self-cleaning effect, the surface must also have a microrough surface structure in addition to being hydrophobic. As early as 1947, the relationship between “fine-particled” surface and high contact angle was described in CH 268 258 A and the analogy with the nasturtium was mentioned. In 1982, A. A. Abramzon in Khimia i Zhizu, Issue No. 11, pages 38-40, published the fact that the relationships of roughness and hydrophobicity are realized, for example in the lotus leaf. Elevations of crystalline type having fine structures which are up to a few μm apart are present on the lotus leaf. Water drops therefore come into contact substantially only with the tips of these elevations, so that the contact area between leaf surface and water drop is tiny. This results in very low adhesion of the water drop. Further such surfaces are known in nature, for example of leaves of nasturtium, on ginkgo leaves and on kohlrabi.
U.S. Pat. No. 3,354,022 discloses water-repellent surfaces, the surface having a microrough structure with elevations and depressions and being formed from a hydrophobic material, in particular from a fluorine-containing polymer. In particular, the production of such surface structures is by embossing a structure on surfaces comprising wax, polymers or wax-like compounds. Furthermore, the production of surface structures by the use of particles is described. Firstly, wax particles are fixed to a surface and secondly glass particles surrounded by wax (fluorohydrocarbon wax based on a fluoroalkylethoxymethacrylate polymer) are fixed by means of a molten wax. In the first case, particles having a size of 5 to 80 μm were used; in the second case, glass particles of from 3 to 12 μm diameter, which agglomerated to give 15 to 250 μm particles, were used. A major disadvantage of such coatings having a self-cleaning effect is the low abrasion resistance thereof.
For improving the abrasion resistance of embossed structures, DE 101 38 036 proposes an embossed structure which has, between the elevations, fins which connect the elevations and have, in the middle between two elevations connected by them, a height which is at least 10% less than the height of the elevations. By means of these measures, the stability of such embossed structures is substantially increased. Usually, the embossing and hence the subject matter of DE 101 38 036 are limited to relatively soft materials, such as, for example, polymers, UV-curable finishes or waxes.
In DE 100 22 246, particles are likewise fixed by means of a film-forming binder, in particular by means of waxes, to surfaces for producing structured surfaces, the particles being enclosed or wetted by the film-forming binders. Owing to the waxes used as binders, there is in this process too the problem that coatings produced in this manner and having a self-cleaning effect have a relatively low abrasion resistance.
WO 96/04123 describes a self-cleaning surface of articles which have an artificial surface structure which has elevations and depressions, the structure being defined by a distance between the elevations of from 5 to 200 μm and a height of the elevations of from 5 to 100 μm. The production of this surface is effected, for example, by applying very finely milled Teflon to a surface treated with adhesive, or by embossing a structure onto thermoplastically deformable hydrophobic material. By fixing the particles with adhesive, better mechanical strength of the structure is achieved than in the case of fixing with waxes.
DE 101 18 352 describes the use of particles for producing a surface structure, it being intended to use particles which have a fine structure in the nanometer range. As a result of the fine structure in the nanometer range, an aspect ratio (maximum height over maximum width of the elevation) of the elevations on the particles of greater than 1 is achieved, as can otherwise be achieved only by embossing. In the case of the processes known to date which use particles which have no structure on the surface, such an aspect ratio of greater than 1 cannot of course be achieved, since maximum height and maximum width are the same in the case of a spherical particle. In DE 101 18 352, the particles are fixed by the use of substrate systems, such as, for example, finishes or adhesives. Although surfaces produced in this manner have a better self-cleaning effect, they are still relatively susceptible to mechanical stressing of the structure. The particles can be very rapidly broken out of the surface, particularly by abrasion against other surfaces.
In DE 101 18 351, mixtures of fixing agent particles and particles having a surface with a fine structure in the nanometer range are used, the fixing agent particles being selected from powder coatings or hot melt adhesives and the fixing being effected by brief heating. Although the surface thus produced exhibits substantially better stability of the particles, there is in this process the danger that the fine structure of the particles will be clogged by fixing agent particles.
It was therefore an object of the present invention to increase the mechanical stability of structured surfaces, it being intended to preserve any surface structure present on the particles themselves.